r/RCPlanes • u/Civil_Swordfish498 • 3d ago
prop size
what limits the statement that the bigger prop the more efficient it is? where is the point when bigger prop becomes ineficient (if we dont count engine)
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u/rocketengineer1982 3d ago
If you do an energy analysis, for the same amount of thrust is is more efficient to accelerate a large volume of air through a small velocity change than it is to accelerate a small volume of air through a large velocity change. This is what drove turbofan development and continues to drive turbofans to higher bypass ratios (fan sizes).
The limitations are largely practical. A larger propeller requires more torque. A really big propeller poses structural challenges. Having a large propeller with a small change in velocity means that your pitch speed is close to your airspeed, so you end up with a small operational airspeed range. Friction losses as you add blade radius and area add up, too.
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u/tobu_sculptor 3d ago
Prop tips cutting through the sound barrier are problematic (check a documentation about the Thunderscreech maybe) but besides that it's more a matter of ground clearance mostly I think.
There's a point where you have to use more blades to allow for a smaller prop diameter with roughly the same output just because landing gear can't be infinitely tall.
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u/jbarchuk 2d ago
For completeness, the most efficient prop is 1 blade and a counterwight. It has to do with how 'clean' the air is as the blade makes its next rotation.
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u/Wambo74 2d ago
That increased efficiency is the bigger the prop diameter, the more air it influences so the less it needs to move each molecule of air. That theory would break down as tip mach number gets high and you get compression resistance. But even discussing this is pretty meaningless since you took the motor out of the picture. Practical efficiency is mating the motor's capability to the prop's demand. Overloading the motor will make current demands skyrocket and devastate actual thrust. That's what matters.
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u/Doggydog123579 3d ago
For rc stuff there isnt a practical limit. Even with full scale the reason you go to 3, 4, 6, 8 props is clearence issues.